Recently, mobile communication terminals, including a portable phone, have come into wide use. From the viewpoint of their portability, it is one of essential factors required to the terminals to be compact and lightweight.
Each electronic element employed in the terminal is also needed to be compact and lightweight to achieve a compact, lightweight terminal. A surface acoustic wave device advantageous for coping with the above requirement, so called a surface acoustic wave filter, has been broadly used for a filter in an intermediate or high frequency circuit of the terminal.
The surface acoustic wave device has a piezoelectric substrate and an interdigital electrode formed on a main surface of the piezoelectric substrate in order to excite, receive, reflect and propagate a surface acoustic wave.
One of important material properties of the piezoelectric substrate used for the surface acoustic wave device is surface wave velocity of the surface acoustic wave (SAW velocity). Two others are temperature coefficient of center frequency in case of its being used for a filter or of resonance frequency in case of its being used for a resonator (TCF), and electromechanical coupling coefficient (k.sup.2).
FIG. 1 shows a table listed each composition of prior piezoelectric substrates generally used for the surface acoustic wave device. The prior piezoelectric substrates shown in FIG. 1 are generally classified into two groups. One group includes 128LN, 64LN and 36LT having electromechanical coupling coefficient and another group includes LT112 and ST quartz having small electromechanical coupling coefficient. 128LN, 64LN and 36LT, which is the piezoelectric substrate having high SAW velocity and high electromechanical coupling coefficient, are applied to the surface acoustic wave filter in the high frequency circuit of the terminal. LT112 and ST quartz, which is the piezoelectric substrate having low SAW velocity and small electromechanical coupling coefficient, are applied to the surface acoustic wave filter in the intermediate frequency circuit of the terminal. The reason for their applications is that the center frequency of the surface acoustic wave filter is directly proportional to the SAW velocity of the applied piezoelectric substrate and is inversely proportional to the electrode finger width of the interdigital electrode formed on the piezoelectric substrate.
Therefore, the substrate having high SAW velocity is suitable for the filter applied to the high frequency circuit. The filter applied to the high frequency circuit of the terminal needs the substrate having high SAW velocity and also high electromechanical coupling coefficient. Because the terminal requires the wideband filter having the passband width of more than 20 MHz.
On one hand, the frequency band ranging from 70 to 300 MHz is used for intermediate frequencies in the mobile terminal. In case that the surface acoustic wave device is used for the filter having the center frequency within the above frequency band, it needs to increase the electrode finger width formed on the substrate in proportion to lowering value of the center frequency considerably compared with that of the filter in the high frequency circuit. This causes a problem that the size of surface acoustic wave device itself becomes large. Therefore, LT112 or ST quartz having low SAW velocity if generally applied to the surface acoustic wave filter in the intermediate frequency circuit. In particular, ST quartz is a suitable material for the piezoelectric substrate due to its zero first order temperature coefficient of frequency. ST quartz can be applied only to the filter having narrow passband due to its property being small electromechanical coupling coefficient. However, up to this time, the above property has hardly caused any problem because the filter in the intermediate frequency circuit has performed only to have one narrow channel signal passed through.
In recent years, in order to used frequency resource effectively and adapt for a digital data communication, a digital mobile communication system has been developed, has been practically established and is also intensively coming into wide use. The digital mobile communication system needs extreme wide passband width ranging from several hundred KHz to several MHz. In case that the surface acoustic wave device is applied to the intermediate frequency filter requiring such wide bandwith, it is impossible to make an adequate substrate of ST quartz. Furthermore, the mounting area of the surface acoustic filter for intermediate frequency is needed to be smaller in order to achieve a smaller mobile terminal having high portability. Through ST quartz and LT112 are recognized as suitable substrates applied to the surface acoustic filter for intermediate frequency, they can hardly contribute to make the substrate smaller due to the fact that their SAW velocity is over 3000 m/sec.